It is difficult, in an electrical transformer, to insulate a very low voltage primary circuit from a very high voltage secondary circuit due to the voltage difference between the two circuits. Another problem with some high voltage isolation transformers is the generation of significant electromagnetic waves at an amplitude and frequency that may interfere with sensitive electronic components. For example, some handheld x-ray fluorescence (XRF) spectrometers require high voltage isolation transformers to provide a small AC signal at a large negative DC potential for the thermionic cathode of an x-ray tube. Electromagnetic waves from these transformers can interfere with an x-ray signal received by an x-ray detector in the XRF spectrometer.
Optimal operation of a transformer is typically at the transformer's resonant frequency. In XRF analyzers using transformers with torroidal shaped cores, electromagnetic waves emitted at the core's resonant frequency may significantly interfere with the operation of the x-ray detector. In addition, the shape of a toroidal transformer can result in a high level of electromagnetic interference (EMI). Shielding and circuit design are often used to mitigate the electromagnetic interference of the detector, but eliminating this interference with circuit design and shielding, especially in the small space available in a relatively small handheld XRF spectrometer, can be difficult.
A toroid shaped core, made of ferromagnetic material, may be used in a high voltage isolation transformer. For example, in handheld XRF spectrometers, the primary windings of the transformer have a relatively low voltage, typically around 10 volts rms AC. The secondary windings carry an alternating current, induced by the AC signal on the primary windings. The secondary windings also have a very large bias voltage of around negative 50,000 volts compared to the primary windings. This bias voltage is generated primarily by a high voltage power supply that is used to apply the bias voltage to the secondary windings. It is very difficult to effectively insulate circuits with such a large voltage difference.
High voltage isolation transformers having a toroid shaped core can have stringent design and manufacturing requirements. To isolate the two widely disparate voltages, of the primary and secondary windings, thick insulation is typically applied to the transformer core, the wire, or both wire and core. Insulation is used that can maintain its integrity and be free of cracks in order to avoid current leakage between the primary and secondary windings. If the bulk of the insulation is on the core, the insulation can crack due to thermal expansion caused by the heating and cooling of the core. One cause of the insulation cracks, during these temperature fluctuations, is a mismatch of the coefficient of thermal expansion (CTE) of the core compared with the CTE of the insulation. Creating this match can be a difficult design challenge. Applying crack-free insulation is often a difficult manufacturing challenge. Thicker insulation can be more difficult to manufacture, without insulation defects, than thinner insulation.